1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system for applying a gradient magnetic field and a radio frequency (RF) pulse to an object to be examined which is placed in a static magnetic field so as to excite a magnetic resonance at a specific portion of the object, acquiring a magnetic resonance (MR) echo signal induced by the magnetic resonance, and obtaining image data such as a slice image of the specific portion and/or analytic data such as spectroscopic data from the MR echo signal data by performing predetermined image reconstruction processing and, more particularly, to adjustment of the transmission power of the RF pulse.
2. Description of the Related Art
In such an MRI system, since a flip angle .theta. of a spin is proportional to the intensity of the RF pulse, as an excitation RF magnetic field normally is applied in the form of a pulse, the RF field intensity must be properly set to set a desired flip angle. In order to control this field intensity, the transmission power of the RF pulse must be properly controlled.
In a conventional system, prior to actual imaging, magnetic resonance is excited in a plane to be imaged while the transmission power of an RF pulse is changed in a predetermined range, and the field intensities of the RF pulse, i.e., MR echo signals, are measured. The RF pulse transmission power at which the maximum peak value of an MR echo signal appears is obtained on the basis of the measurement result. The transmission power of the RF pulse for imaging is controlled on the basis of the RF pulse transmission power having the maximum peak value so as to set a desired flip angle of a spin. Such control is called automatic power control.
In conventional automatic power control, however, if a plane to be imaged is a sagittal plane (a vertical plane extending along the body axis) or a coronal plane (a horizontal plane extending along the body axis) other than a transaxial plane (perpendicular to the body axis), the following problems are posed. The field intensity of an RF pulse transmitted from an RF transmission coil is uniform only in a predetermined region covered within an internal space of a transmission coil, and this region is defined as an imaging region. However, since an RF transmission coil and an RF reception coil (sometimes a single RF coil may serve as these two coils) are sensitive in a wide range outside the imaging region, a region excited by the RF pulse includes a portion outside the imaging region, in which the RF field intensity is not uniform.
It is assumed, as shown in FIG. 1, that an object P is placed in an RF coil 1 as a transmission/reception coil so as to perform imaging of an abdominal portion P1 of the object P. Although the abdominal portion P1 is placed in the imaging region (hatched portion), if the object is a person other than infants, the legs and the head are placed outside the transmission coil 1. For this reason, when a tomographic image of a sagittal plane or a coronal plane is to be obtained, regions (legs and head) having nonuniform RF field intensities, other than the imaging region are also excited and MR echo signals therefrom are acquired. When a transaxial plane is to be imaged, since it includes no portions, of the object located outside the imaging region, no MR echo signals from the outside the imaging region are acquired.
If an MR echo signal obtained in this case, e.g., a spin echo signal, is represented by S(t), with projection data F(.omega.) obtained by performing a Fourier transform of S(t), the projection data F(.omega.) is represented as follows: EQU F(.omega.)=.intg.S(t)e.sup.i.omega.t dt (1)
It is noted that the z direction is a read direction in this case.
FIG. 2 shows a relationship between the projection data F(.omega.) and .omega.. If equation (1) is subjected to an inverse Fourier transform, the spin echo signal S(t) is represented as follows: EQU S(t)=a.intg.F(.omega.)e.sup.-i.omega.t d.omega. (2)
where a is a constant.
FIG. 3 shows the relationship between the spin echo signal S(t) and t. According to equation (2), a peak value So of the spin echo signal, i.e. the signal at t=0, corresponds to the area of the projection data F(.omega.) in the following manner: EQU So=a.intg.F(.omega.)d.omega. (3)
When the sagittal plane or the coronal plane of the abdominal portion P1 in FIG. 1 is to be imaged, the projection data F(.omega.) obtained includes signal components from portions outside the imaging region in addition to a signal component P2 corresponding to only the abdominal portion P1, as shown in FIG. 2. As a result, the spin echo signal So obtained by integrating the data F(.omega.) inevitably includes signals from outside the imaging region.
As described above, in the conventional automatic power control system, when a condition in which a spin echo having the maximum or minimum peak value appears, is to be obtained, regions having nonuniform field intensities and located outside an imaging region are excited, and signals from these regions are also acquired. Therefore, an optimal transmission power, i.e. an optimal transmission power condition for a 90.degree. or 180.degree. pulse, cannot be obtained. For this reason, the flip angle of a spin deviates from a preset value, and the contrast of the image obtained is deteriorated.